In vehicles on the market now, various electronic control devices to improve driver convenience and safety are provided and an in-vehicle communication network for communication between these electronic control devices is basically provided.
Particularly, as the number of in-vehicle electronic controllers continuously increases and the electronic controllers can interwork with various external devices, overload of a conventional in-vehicle communication network and costs for wiring harnesses increase.
Further, according to consumer requirements for high-quality images and audio data and increase in image using applications, necessity for bandwidth extension increases.
Therefore, many vehicle manufacturers consider introduction of Ethernet as an in-vehicle communication network, and some vehicle manufacturers mass-produce an Around View Monitor (AVM) system based on Ethernet.
General Ethernet includes a plurality of Local Area Networks (LANs) and a plurality of bridges for interconnection between the LANs.
Ethernet is characterized in that a plurality of nodes competitively attempts to approach a common medium using a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol. However, since the same priority is given to all traffic and the nodes transmit their own traffic through competition, the CSMA/CD protocol is not suitable for transmission of multimedia data sensitive to transmission time delay, such as moving pictures and audio data.
Therefore, Ethernet uses time synchronization of all nodes on a network for transmission of multimedia data sensitive to transmission time delay.
In transmission protocol layer standards, such as IEEE 1722, an Audio Video Bridging (AVB) standard for transmission of audio or video signals sensitive to a streaming time is being developed now. In the AVB standard, transmission quality guaranteed technology to effectively transmit a multimedia stream, such as audio or video data, on Ethernet is to be developed.
As described above, the conventional LAN, particularly, representative technology thereof, i.e., Ethernet, fundamentally uses frame-based packet switching and thus has a difficulty in providing effective quality guaranteed transmission. In order to solve such a drawback, AVB named Synchronous Ethernet or Residential Ethernet in IEEE 802.2 has been first developed, and a method of implementing similar technology on bridges to the extent of not significantly affecting the paradigm of conventional nonsynchronization packet switching in IEEE 802.1 is being developed now.
Fundamentally, the AVB technology in IEEE 802.1 enables synchronous traffic transmission using conventional Ethernet bridges performing packet switching and it is important that clocks of bridges within a designated geographical range are synchronized. When clocks of the bridges are synchronized, an Ethernet frame having a regular size may be transmitted between the bridges by designated time intervals at an accurately desired time. Therefore, a bridge mesh to which such a fundamental concept is applied may be used as infrastructure for stably transmitting synchronous traffic.
For example, IEEE 1588 Precision Time Protocol (PTP) is a time synchronization standard operable throughout all Open Systems Interconnection (OSI) layers and IEEE 802.1AS is a time synchronization standard supporting the profile of only OSI layer 2, i.e., a data link layer. If IEEE 802.1AS is applied to layer 2 devices, such as bridges and switches, an OSI layer 2 time synchronization network may be configured.
In a time synchronization method between respective devices in IEEE 802.1AS, a transmission side and a reception side are synchronized using time stamps including time synchronization information and, in order to perform time synchronization, a grandmaster (GM) to provide a reference time is selected from among devices in a network, and the local time of the selected grandmaster is transmitted to other devices through an announcement message so that the devices use the local time as the reference time. Particularly, a transmission delay time between a point of transmission time of a specific message at a transmission terminal and a point of reception time of the message at a reception terminal is measured and thereby, the reference time of the reception terminal is corrected. Here, the grandmaster transmits the announcement message to all of other devices and thus transmits its own presence and a comparison value for validity as a device providing a reference time to these devices.
That is, the grandmaster is the uppermost node of an IEEE 802.1AS timing tree and periodically transmits current time information to lower nodes.
In IEEE 802.1AS, a procedure for determining a grandmaster and acquiring time synchronization, a procedure for searching all devices in a network using a plurality of control messages and controlling approach to links, a procedure for continuously confirming link states through announcement messages, and the like are defined.
However, in conventional IEEE 802.1AS, if a bridge, serving as the grandmaster, is not normally operated, a new grandmaster is set again through a Best Master Clock Algorithm (BMCA). Therefore, the amount of traffic on the Ethernet network increases and it takes a long time to reconfigure time synchronization on a system.